Aldehyde dehydrogenase class 3 (ALDH3) comprises approximately 40 percent of the cellular protein of the mammalian corneal epithelial cells, an amoun reminiscent of an enzyme-crystallin in the lens. Consequently, we are investigating the molecular basis for the high expression of the ALDH3 gene in the corneal epithelial cells. The results will be compared with those obtained for crystallin genes in the lens and will provide a foundation for eventual gene therapy in the cornea. The complete mouse ALDH3 protein has been deduced from a cloned corneal complementary deoxyribonucleic acid (cDNA). Three 16-18 kbp mouse genomic fragments for ALDH3 have been cloned: one comprises the entire ALDH3 gene, one contains about 13 kbp of 5~ flanking sequence, exon 1, intron 1 (3.2 kbp) and part of exon 2, and th third is being analyzed. Northern blots have established that ALDH3 messenger ribonucleic acid (mRNA) is at least 100 times more prevalent in the cornea than in the stomach, bladder, and lung, the only other tissues showing a trace of this gene product. Transfection and transgenic mouse experiments using the chloramphenicol acetyltransferase (CAT) reporter gene have shown that 1050 bp of 5~ flanking sequence of the ALDH3 gene gives low-level expression in the liver, but is inactive in all other tissues tes d, including the cornea. New promoter/CAT constructs containing intron 1 of the ALDH3 gene have been made and are being tested. The 5~ flanking sequence of the ALDH3 gene contains numerous potential control elements, including antioxidant response elements, which will be tested for function.